This invention relates to a taper-machining method for use in a wire cut electric discharge machining apparatus in which a workpiece is machined with a wire electrode inclined to form a tapered surface in the workpiece.
FIG. 1 is an explanatory diagram illustrating the arrangement of a conventional wire cut electric discharge machining apparatus. In FIG. 1, reference numeral 1 designates a wire-shaped electrode; 2, an upper wire guide for guiding the wire-shaped electrode 1; 3 and 4, drive units for driving the wire guide 2 in a U-axis direction and in a V-axis direction, respectively; 5, a lower wire guide; 6, a wire supplying reel; 8, a wire winding reel; 9, a wire winding guide roller; 10, a workpiece supported by a movable table 11 between the upper and lower wire guides 2 and 5; and 12 and 13, drive units for driving the movable table 11 in an X-axis direction and in a Y-axis direction, respectively.
FIGS. 2(a) through 3(c) show examples of a workpiece which has been machined by the wire cut electric discharge machining apparatus described above. More specifically, FIGS. 2(a) through 2(c) show an example of a workpiece which has been taper-machined into a substantially circular truncated cone (hereinafter referred to as "a first taper-machining method", when applicable). In contrast, FIGS. 3(a) through 3(c) show an example of a workpiece which has been taper-machined according to a method of taper-machining a workpiece in such a manner that it has the same radius at the corners of the upper and lower surfaces thereof (hereinafter referred to as "a second tapering-machining method", when applicable). In these figures, the parts (a), (b) and (c) are a top view, a front view, and a bottom view showing the taper-machined workpiece, respectively.
FIG. 4 is a schematic diagram illustrating a sectional view of the workpiece being taper-machined. In FIG. 4, reference character t represents the thickness of a workpiece 10; .theta..sub.1, the inclination angle of the wire-shaped electrode 1 with respect to the vertical line (hereinafter referred to as "a wire inclination angle", when applicable); and t.sub.i, the horizontal distance between the machining locus of the upper surface 10a of the workpiece 10 and that of the machining locus of the lower surface 10b. The horizontal distance, corresponding to the wire inclination angle .theta..sub.i and the thickness t of the workpiece 10, represents the taper offset.
FIG. 5 is a schematic diagram showing a machining loci in the aforementioned second taper-machining method. In FIG. 5, reference numeral 20 designates a first straight part; 21, an arcuate part merging with the first straight part; and 22, a second straight part merging with the arcuate part 21. Further in FIG. 5, reference character l.sub.1, c.sub.1 and l.sub.2 designates a first straight locus, an arcuate locus, and a second straight locus, respectively, which define the lower source 10b of the workpiece; and l.sub.3, c.sub.2, and l.sub.4, a first straight locus, an arcuate locus, and a second straight locus, respectively, which define the upper surface 10a of the workpiece. Further in FIG. 5, reference character O.sub.1 and O.sub.2 designate the centers of the arcuate loci c.sub.1 an c.sub.2, respectively; r, the radius of the arcuate loci c.sub.1 and c.sub.2 ; and t.sub.1 and t.sub.2, the taper offsets of the first and second straight parts 20 an 22, which are represented by the following equations, respectively: EQU t.sub.1 =t.times.tan .theta..sub.1 EQU t.sub.2 =t.times.tan .theta..sub.2
where t is the thickness of a workpiece, and .theta..sub.1 and .theta..sub.2 are the wire inclination angles of the first and second straight parts, respectively.
Further in FIG. 5, reference characters g.sub.1 and g.sub.2 designate grooves formed in the upper and lower surfaces of the workpiece when the wire-shaped electrode 1 is moved.
The operation of the wire cut electric discharge machining apparatus will be described.
In a wire cut electric discharge machining operation, as is well known in the art, first the drive units 12 and 13 are operated to drive the movable table 11 to move the workpiece 10 with respect to the wire-shaped electrode 1, to machine it as required. On the other hand, by operating the drive units 3 and 4 of the upper wire guide 2 in synchronization with the drive units 12 and 13 of the movable table 11, a sloped surface can be formed on the workpiece; that is, the latter is taper-machined. This taper-machining method is practiced as the aforementioned first taper-machining method in which a workpiece is taper-machined into a substantially circular truncated cone, or as the aforementioned second taper-machining method in which a workpiece is taper-machined to have the same radius at the corners of the upper and lower surfaces. In the second taper-machining method, the machining grooves are constant in the upper and lower surfaces of the workpiece 10 (because the corners of the upper and lower surfaces are machined at the same speed, and accordingly in a given period of time the amount of machining of the upper surface is equal to the amount of machining of the lower surface). Hence, the second taper-machining method is essential for provision of taper angles with high accuracy.
The second taper-machining method will be described with reference to FIG. 5 in more detail.
As is disclosed, for instance, by Published Examined Japanese Patent Application No. 49053/1961, in the second taper-machining method, in order to obtain the arc of the workpiece upper surface which corresponds to the arcuate part 21 merging with the first and second straight parts 20 and 22 of the workpiece upper surface, first the first and second straight loci l.sub.3 and l.sub.4 of the workpiece upper surface are obtained which are spaced the taper offsets t.sub.1 and t.sub.2 from the first and second straight loci l.sub.1 and l.sub.2, respectively, and then the arcuate locus c.sub.2 of the workpiece upper surface which is tangent to the loci l.sub.3 and l.sub.4 is obtained. Therefore, in the arcuate loci c.sub.1 and c.sub.2 of the workpiece upper and lower surfaces, with the arc start point A.sub.1 and A.sub.2 referred to each other, and with the arc end points B.sub.1 and B.sub.2 referred to each other, the first straight part 20 is machined. That is, the wire inclination angle is gradually changed until the wire-shaped electrode 1 reaches the arc start point A.sub.1 and A.sub.2 of the arc loci c.sub.1 and c.sub.2 at the same time. After the wire-shaped electrode 1 has reached the arc start points A.sub.1 and A.sub.2, with the wire inclination angle maintained unchanged the taper-machining operation for formation of the arcuate part 21 is continued until the wire-shaped electrode 1 moving along the arcuate loci c.sub.1 and c.sub.2 (having the same radius) reaches the arc end points B.sub.1 and B.sub.2 thereof. Thereafter, for the second straight part 22, the taper-machining operation is started from the arc end points B.sub.1 and B.sub.2 while the wire inclination angle being changed gradually. Thus, in the workpiece, the arc machining loci of the upper and lower surface are equal to each other, and accordingly the machining speeds of the upper and lower surfaces are equal, and the machining grooves are constant in width.
The second taper-machining method disclosed by the above described Publication No. 49053/1986 is provided only for the arc of the corner part between two straight parts which is tangent to the two straight lines (hereinafter referred to as "a tangent-circle-arc", when applicable); that is, the method can not be employed without obtaining the points of the straight lines to which the arc is tangent, and the center position and the radius of the arc. In practice, sometimes it is required to machine a workpiece to form a configuration, such as a curve bent inwardly or outwardly, which is different from the tangent-circle-arc (hereinafter referred to as "a non-tangent-circle-arc", when applicable). In this case, it is impossible for the above-described machining technique to univocally define the connection points of the arc and the straight lines, and the center position and the radius of the arc. Therefore, in machining a workpiece to form a non-tangent-circle-arc by the second taper-machining method, other intricate taper-machining methods in which the upper and lower surfaces are optional in configuration must be employed.